I know verb-んじゃない can be used in colloquial speech as a negative imperative (e.g. するんじゃない) in lieu of verb-な (するな), but why exactly does it function that way? That is, what is the rationale/etymology behind v-んじゃない being a negative imperative?
Obviously it's not grammatical imperative, but the construction functions as order when used by somebody's betters (senior, superior etc.) to strongly admonish them. If I can ignore context, "You don't want to do —!" could be a way of translation.
- V(する)-んだ！ (more pompously V(する)-のだ！): affirmative command
- V(する)-んじゃない！ (V(する)-のではない！, V(する)-でない！): negative command
Incidentally, the dictionary form is also employed as command, with colloquial but very overbearing tone.
- V(する)！: affirmative
- V(し)-ない！: negative